Schools of skyrmions with electrically tunable elastic interactions

Abstract

Coexistence of order and fluidity in soft matter often mimics that in biology, allowing for complex dynamics and applications-like displays. In active soft matter, emergent order can arise because of such dynamics. Powered by local energy conversion, this behavior resembles motions in living systems, like schooling of fish. Similar dynamics at cellular levels drive biological processes and generate macroscopic work. Inanimate particles capable of such emergent behavior could power nanomachines, but most active systems have biological origins. Here we show that thousands-to-millions of topological solitons, dubbed “skyrmions”, while each converting macroscopically-supplied electric energy, exhibit collective motions along spontaneously-chosen directions uncorrelated with the direction of electric field. Within these “schools” of skyrmions, we uncover polar ordering, reconfigurable multi-skyrmion clustering and large-scale cohesion mediated by out-of-equilibrium elastic interactions. Remarkably, this behavior arises under conditions similar to those in liquid crystal displays and may enable dynamic materials with strong emergent electro-optic responses.

Optically enriched and guided dynamics of active skyrmions

“…1e. Similar asymmetric skyrmions have been demonstrated in both LCs and chiral magnets [9,37,[44][45][46][47]. Due to their topological stability, the resulting asymmetric skyrmions maintain their topological nature under tunable deformation [9,36,37].…”

“…Because our LC skyrmions represent minima in the elastic free energy of our chiral nematic experimental systems, we minimize the Frank-Oseen free energy to computer simulate the n(r) of a single skyrmion at electric fields E corresponding to different applied voltages U: [9,20,35,36]…”

“…This morphing of the director field with electric field application is not invariant upon reversal of time, which results in a net translational motion upon voltage modulation [36]. Many skyrmions under such energy conversion conditions tend to exhibit long-range interactions and schooling behavior [9]. Vectorized n(r) plots ( Fig.…”

“…However, this electricallyinduced active motion has only limited types of collective emergent behavior. Previous developments only allow for one "setting" of motion where all skyrmions tend to synchronize to move together at a constant velocity and in the same direction [9]. Inspired by complex dynamics that one observes in crowds of people and other active systems interacting with various obstacles and environments, we present an experimental approach for selective control of skyrmion motion using diverse optical manipulation techniques.…”

Electrically powered motions of toron crystallites in chiral liquid crystals

“…The solitons were generated by first inducing and then relaxing electrohydrodynamic instability (SI Fig. S1) obtained at U=20 V and f=2 Hz, forming spontaneously as energetically favorable structures after turning U off because of the chiral LC's tendency to twist [24]. By manually switching on and off U that induces the hydrodynamic instability 3-5 times in a matter of a few seconds, one can increase the number density as desired, up to tight packing of torons.…”